The present application relates to the field of radiographic imaging. It finds particular application with computed tomography (CT) scanners. It also relates to medical, security, and other applications where generating a colored two-dimensional projection image from three-dimensional image data would be useful.
CT and other radiographic imaging systems are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation, and a two-dimensional image and/or a three-dimensional image is formed based upon the radiation absorbed by the interior aspects of the object, or rather an amount of radiation that is able to pass through the object. Typically, highly dense aspects of the object absorb more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or mass, for example, will be apparent when surrounded by less dense aspects, such as fat tissue or muscle.
In three-dimensional imaging scanners, such as CT scanners, for example, three-dimensional colored images may be produced from data that is acquired from a radiological examination of an object. Generally, the colors are representative of a property of the object, or rather a property of interior aspects of the object, such as density and/or atomic number. For example, inside a suitcase under examination, a metal plate (e.g., with a density of 11340 kg/m3) may be colored red while a plastic box (e.g., with a density of 1000 kg/m3) may be colored yellow. A colored three-dimensional image may be projected onto an arbitrary observation plane through a technique commonly referred to as “rendering.” The resulting colored image can be displayed on the computer screen. In an airport security environment, for example, coloring the image may assist security personnel in identifying objects that may be a threat (e.g. guns, knives, explosives, etc.).
Three-dimensional image scanners may also display two-dimensional projection images of an object under examination. In some scanners, these two-dimensional images can be formed directly from the acquired data, without forming a 3D image. Because three-dimensional images take longer to generate and may be more time consuming for human personnel to review, in some applications (e.g., high throughput luggage examinations) it may be more beneficial for personnel to initially review a two-dimensional projection image. If the personnel believes a closer examination is warranted (e.g., a potential security threat exist in a piece of luggage), the personnel may consult a three-dimensional image of the object for a more detailed review of the object.
While two-dimensional projection images have a high resolution and low artifacts (e.g., in comparison to two-dimensional synthetic images produced by rendering three-dimensional images, that typically comprise more artifacts and/or are more grainy), a disadvantage of two-dimensional projection images generated from three-dimensional image scanners is that they are often formed as grayscale images (e.g., the images lack color) because the image pixels carry information on a total attenuation value along a particular direction, and not on specific density values encountered along said direction. Thus, the interior aspects of the object will appear in varying shades of gray depending upon the aspects' attenuations.
In some three-dimensional scanners, two-dimensional projection images may be colored similarly to projection images produced by two-dimensional scanners (e.g., line scanners). That is, the two-dimensional projection image may be colored based upon the attenuation of radiation that traversed the object under examination (e.g., a shade of gray on the projection image is replaced with a color indicative of the shade of gray it is replacing). For example, a first portion of the object attenuating fifty percent of the radiation that traversed the first portion may be colored blue on the projection image, and a second portion of the object, attenuating twenty percent of the radiation that traversed the second portion may be colored yellow on the projection image. While such coloring has proven useful, the coloring does not reliably depict the composition of aspects of the object under examination. For example, a stack of books may be colored the same color as a metal plate because both a stack of books and a metal plate may attenuate a similar amount of radiation. As can be appreciated, this can inhibit an ability to accurately examine one or more scanned objects.